1. Field of the Invention
The present invention generally relates to a light guide plate, an illuminating device and a display device having the same. More particularly, the present invention relates to a light guide plate preferably used as a frontlight of a reflection-type liquid crystal display device (LCD), an illuminating device and a display device having the same.
2. Description of the Background Art
Liquid crystal display devices (LCDs) are thin, low power consumption display devices. Because of such characteristics, the LCDs are recently used in a variety of applications including office automation (OA) equipments such as word processors and personal computers, portable information equipments such as electronic organizers, and camera-incorporated VTRs (video tape recorders) having a liquid crystal monitor.
These LCDs are roughly divided into two types: reflection type and transmission-type. An LCD is not a self-light-emitting display device such as CRT (cathode ray tube) and EL (electroluminescence). For display, a transmission-type LCD uses light of an illuminating device provided behind a liquid crystal display (LCD) panel (so-called backlight), whereas a reflection-type LCD uses ambient light.
Some known reflection-type LCDs have an illuminating device for improving display quality when ambient light having a sufficient intensity is not available. This illuminating device is called xe2x80x9cfrontlightxe2x80x9d, while the illuminating device of the transmission-type LCDs is called xe2x80x9cbacklightxe2x80x9d.
Various illuminating devices are conventionally proposed as such a backlight and frontlight. For example, Japanese Laid-Open Publication No. 8-94844 discloses an illuminating device 400 used as a backlight and frontlight, as schematically shown in FIG. 38. Hereinafter, the structure of a reflection-type LCD 450 having the illuminating device 400 as a frontlight will be described with reference to FIG. 38.
The reflection-type LCD 450 has a reflection-type LCD panel 430 and an illuminating device (frontlight) 400 provided on the viewer side of the reflection-type LCD panel 430. The illuminating device 400 has a light source 410 and a light guide plate 420. A reflecting member 412 surrounds the light source 410.
The light guide plate 420 has a first side surface (incident side surface) 422a located at the light source 410, a second side surface (opposing side surface) 422b facing the first side surface 422a, third and fourth side surfaces (not shown), an emitting surface 422c for emitting the incident light from the light source 410, and an opposing surface 422d facing the emitting surface 422c. The third and fourth side surfaces are located between the first side surface 422a and the second side surface 422b so as to face each other. The reflection-type LCD panel 430 is mounted so as to face the emitting surface 422c of the light guide plate 420. The opposing surface 422d has propagating surfaces 422d1 and reflecting surfaces 422d2, and has a saw-tooth cross-section.
Light emitted from the light source 410 into the light guide plate 420 through the first side surface 422a is propagated toward the second side surface 422b while being repeatedly totally reflected by the propagating surfaces 422d1 and the emitting surface 422c. The light propagated through the light guide plate 420 is partially reflected by the reflecting surfaces 422d2 and emitted from the emitting surface 422c toward the reflection-type LCD panel 430. The light reflected by the reflection-type LCD panel 430 is directed back into the light guide plate 420 through the emitting surface 422c. After passing through the light guide plate 420, the reflected light is emitted from the opposing surface 422d toward the viewer for use in display.
Japanese Laid-Open Publication No. 10-268307 discloses an illuminating device 500 used as a frontlight, as schematically shown in FIG. 39A. Hereinafter, the structure of a reflection-type LCD 550 having the illuminating device 500 as a frontlight will be described with reference to FIG. 39A.
The reflection-type LCD 550 has a reflection-type LCD panel 530 and an illuminating device (frontlight) 500 provided on the viewer side of the reflection-type LCD panel 530. The illuminating device 500 has a light source 510 and a light guide plate 520. A reflecting member 512 surrounds the light source 510. A condenser 513 is provided between the light source 510 and the light guide plate 520.
The light guide plate 520 has an incident side surface 520a located at the light source 510, an emitting surface 520b facing the reflection-type LCD panel 530, an opposing surface 520c located on the viewer side and facing the emitting surface 520b, and first and second side surfaces (not shown). The first and second side surfaces face each other with the incident side surface, emitting surface and opposing surface interposed therebetween. The light guide plate 520 has a wedge-like shape so that the distance between the emitting surface 520b and the opposing surface 520c is reduced as the distance from the incident side surface 520a is increased. The light guide plate 520 has a triangular cross section along the direction perpendicular to the emitting surface 520b and the opposing surface 520c. The incident side surface 520a is tilted with respect to the normal to the emitting surface 520b, and forms an obtuse angle with the emitting surface 520b. 
Light emitted from the light source 510 into the light guide plate 520 through the incident side surface 520a is propagated within the light guide plate 520. Of the light propagated within the light guide plate 520, the light that does not satisfy the conditions for total reflection at the emitting surface 520a is emitted from the emitting surface 520b toward the reflection-type LCD panel 530. The light reflected by the reflection-type LCD panel 530 is directed back into the light guide plate 520 through the emitting surface 520b. After passing through the light guide plate 520, the reflected light is emitted from the opposing surface 520c toward the viewer for use in display.
Japanese Laid-Open Publication No. 10-268307 discloses a reflection-type LCD 550xe2x80x2 having an illuminating device 500xe2x80x2 used as a frontlight, as schematically shown in FIG. 39B. The reflection-type LCD 550xe2x80x2 is different from the reflection-type LCD 550 in that the reflection-type LCD 550xe2x80x2 has a light traveling direction controller 524. In the reflection-type LCD 550xe2x80x2, the light traveling direction controller 524 is provided so as to face the emitting surface 520b of the light guide plate 520. Therefore, as the light emitted from the emitting surface 520b passes through the light traveling direction controller 524, the traveling direction thereof is changed toward the direction normal to the emitting surface 520b. 
However, the inventor found that the illuminating devices disclosed in the above publications have the following problems:
In the illuminating device 400 disclosed in the aforementioned Japanese Laid-Open Publication No. 8-94844, the propagating surfaces 422d1 and the reflecting surfaces 422d2 form concaves and convexes at the opposing surface 422d of the light guide plate 420. Therefore, any damage to such concaves and convexes or adhesion of foreign matters thereto would adversely affect the illumination light, degrading display quality of the LCD. When the illuminating device 400 is used as a frontlight, the opposing surface 422d is located on the viewer side. Therefore, such problems are especially likely to occur, and the reflection-type LCD 450 having the illuminating device 400 as a frontlight as shown in FIG. 38 would be subjected to significant degradation in display quality.
In order to prevent such damages to the concaves and convexes and adhesion of foreign matters thereto, a display device 600 mounted in a portable terminal (RZ-J90) made by SANYO Electric Co., Ltd. has a protection plate 630 on the front (viewer side) of a light guide plate 620, as schematically shown in FIG. 40A.
As schematically shown in FIG. 40B, a display device 700 mounted in a portable terminal (MI-310) made by Sharp Corporation has a touch panel 730 on the front (viewer side) of a light guide plate 720. In the display device 700, the touch panel 730 functions as the above protection plate. Typically, the touch panel 730 is a lamination of a lower electrode substrate 731 and an upper electrode film 736 with an adhesive portion 733 interposed therebetween. The lower electrode substrate 731 has a lower electrode 732 of a transparent conductive film and a spacer 738 thereon. The upper electrode film 736 has an upper electrode 734 of a transparent conductive film thereon. When information is input to the touch panel 730, the upper electrode film 736 is pressed and the upper electrode 734 is electrically connected to the lower electrode 732 according to deformation of the pressed upper electrode film 736.
However, when a protection plate or touch panel is provided on the front of the light guide plate as in the above examples, surface reflection of the protection plate or touch panel degrades display quality of the display device. Moreover, the overall thickness of the display device is increased.
On the other hand, in the illuminating device 500 disclosed in the aforementioned Japanese Laid-Open Publication No. 10-268307, the light guide plate 520 does not have any concaves and convexes at its surface. Therefore, the illumination light will not be adversely affected by damages thereto and the like.
In this illuminating device 500, however, the light emitted from the emitting surface 520b does not travel in the direction perpendicular to the emitting surface 520b. Accordingly, the reflection-type LCD panel 530 cannot be effectively illuminated. By increasing the angle between the emitting surface 520b and the opposing surface 520c, the traveling direction of the emitted light can be made closer to the direction normal to the emitting surface 520b. However, this increases the thickness of the light guide plate 520 and thus increases the overall thickness of the display device.
The illuminating device 500xe2x80x2 disclosed in the aforementioned Japanese Laid-Open Publication No. 10-268307 has the light traveling direction controller 524 capable of making the traveling direction of the light emitted from the emitting surface 520b closer to the direction normal to the emitting surface 520b. However, the light traveling direction controller 524 also changes the traveling direction of the light emitted from the reflection-type LCD 530 when the emitted light passes therethrough. As a result, the reflection-type LCD 550xe2x80x2 having the illuminating device 500xe2x80x2 cannot effectively display an image.
The present invention is made in view of the above problems, and it is an object of the present invention to provide a reliable light guide plate, an illuminating device and a display device having the same.
According to one aspect of the present invention, an illuminating device includes a light source and a light guide plate. The light guide plate has a first side surface receiving light emitted from the light source, a second side surface facing the first side surface, a third side surface, a fourth side surface, an emitting surface and an opposing surface, the third and fourth side surfaces are located between the first and second side surfaces so as to face each other, and the emitting surface and the opposing surface face each other with the first, second, third and fourth side surfaces interposed therebetween. The light guide plate is formed from a first light guide layer and a second light guide layer each having a prescribed thickness in a direction normal to the emitting surface. The first light guide layer has a plurality of reflecting films therein for reflecting light entering the light guide plate through the first side surface toward the emitting surface. The second light guide layer has no reflecting film. The above object is thus achieved.
Preferably, an angle xcex1 between a tangent line of the reflecting film and the emitting surface and a critical angle xcex8 of the light guide plate satisfy the following relation:
45xc2x0xe2x88x92xcex8/2 less than xcex1 less than 45xc2x0+xcex8/2.
Preferably, an angle xcex1 between a tangent line of the reflecting film and the emitting surface and a critical angle xcex8 of the light guide plate satisfy the following relation:
xcex8 less than xcex1 less than 45xc2x0+xcex8/2.
Preferably, a light transmittance of the light guide plate is at least 85%.
Preferably, a sum xcexa3xcex94s of respective areas xcex94s of the reflecting films projected on the emitting surface, a reflectance r of the reflecting film, and an area S of the emitting surface of the light guide plate satisfy the following relation:
0 less than (xcexa3xcex94s/S)xc2x7rxe2x89xa60.08.
The reflecting films may have a line shape when viewed from the direction normal to the emitting surface.
The reflecting films may have an island shape when viewed from the direction normal to the emitting surface.
The reflecting films may be transparent dielectric films or dielectric multi-layer films.
Preferably, provided that a principal ray of the light entering the light guide plate through the first side surface is incident on the dielectric films or the dielectric multi-layer films at an incident angle xcex2, a refraction angle xcex3 of the principal ray in the dielectric film or the dielectric multi-layer film, a thickness d of the dielectric film or the dielectric multi-layer film, a refractive index n of the dielectric film or the dielectric multi-layer film, and a refractive index n0 of the first light guide layer satisfy the following relation:
380/(4nxc2x7cos xcex3)xe2x89xa6dxe2x89xa6780/(4nxc2x7cos xcex3), and
n0xc2x7sin xcex2=nxc2x7sin xcex3.
The light guide plate may be a lamination of a first light guide element and a second light guide element with an adhesive layer interposed therebetween. In this lamination, the first light guide element, the adhesive layer and the second light guide element are laminated in the direction normal to the emitting surface. A surface of the first light guide element which faces the adhesive layer may have a plurality of tilted regions that are tilted with respect to the emitting surface. The reflecting films may be formed on the tilted regions of the surface.
Preferably, the surface of the first light guide element which faces the adhesive layer further has a plurality of vertical regions that extend approximately perpendicularly to the emitting surface, and the tilted regions and the vertical regions are alternately arranged in a prescribed direction.
Preferably, a width A of the tilted region of the surface in the prescribed direction and a width a of the reflecting film in the prescribed direction satisfy the following relation:
0 less than axe2x89xa6A.
The reflecting films may be formed over a whole surface of the tilted regions of the surface.
The surface of the first light guide element which faces the adhesive layer may further have a plurality of vertical regions that extend approximately perpendicularly to the emitting surface and a plurality of parallel regions that extend approximately in parallel with the emitting surface. Typically, the tilted regions, the vertical regions and the parallel regions are arranged in prescribed order along a prescribed direction in a cyclic manner. When the surface of the first light guide element which faces the adhesive layer is located near the emitting surface of the light guide plate, the parallel regions are preferably located closer to the emitting surface than are the tilted regions. When the surface of the first light guide element which faces the adhesive layer is located near the opposing surface of the light guide plate, the parallel regions are preferably located closer to the opposing surface than are the tilted regions.
Preferably, the first light guide element and the adhesive layer have approximately the same refractive index.
The light guide plate may be a lamination of a first light guide element and a second light guide element. In this lamination, the first light guide element and the second light guide element are laminated in the direction normal to the emitting surface. A surface of the first light guide element which faces the second light guide element may have a plurality of tilted regions that are tilted with respect to the emitting surface. The reflecting films may be formed on the tilted regions of the surface.
The second light guide element may be located on a viewer side of the first light guide element and function also as a transparent input device.
According to another aspect of the present invention, a display device includes an illuminating device having the above structure and a reflection-type display panel facing the light guide plate of the illuminating device on a side opposite to a viewer. The above object is thus achieved.
According to still another aspect of the present invention, a light guide plate has a first side surface receiving light emitted from a light source, a second side surface facing the first side surface, a third side surface, a fourth side surface, an emitting surface and an opposing surface. The third and fourth side surfaces are located between the first and second side surfaces so as to face each other, and the emitting surface and the opposing surface face each other with the first, second, third and fourth side surfaces interposed therebetween. The light guide plate is formed from a first light guide layer and a second light guide layer each having a prescribed thickness in a direction normal to the emitting surface. The first light guide layer has a plurality of reflecting films therein for reflecting light entering the light guide plate through the first side surface toward the emitting surface. The second light guide layer has no reflecting film. The above object is thus achieved.
According to the present invention, a reliable illuminating device is provided, and a display device capable of providing high quality display is provided by using the illuminating device.
The illuminating device of the present invention can be preferably used as a backlight or frontlight of an LCD. In the illuminating device of the present invention, illumination light having desired characteristics is obtained by appropriately setting arrangement of the reflecting films within the light guide plate. In particular, the illuminating device of the present invention is preferably used as a frontlight of a reflection-type display device.